Combined effects of salts and nitrogen on the yield function of lettuce

The objective of this work is to study the combined effects of saline water and nitrogen on the yield function of Summer lettuce (Longifolia Lam., cv. Nevada), under an experimental design known as triple emitter source TES. Three trickle laterals were connected together in order to form a triple joint lateral. Two of them and their emitters were connected to two tanks of stock solutions. One tank contained a solution of NaCl and the other of NH ₄NO₃. The third lateral contained only fresh water, where electrical conductivity was 0.9 dS m^-1. The emitters on the three coupled lines had different discharges, varying their concentrations, but the accumulative discharges for each dripping point were constant. Maximal concentrations of NaCl and NH₄NO₃ were, respectively, 7 and 2 g L^-1, being the maximal accumulative amounts of NaCl and NH₄NO₃ per plant, respectively120 g plant^-1 (S₁ = 10 g plant^-1; S₂ = 50 g plant ^-1; S₃ = 75 g plant^-1; S₄ = 100 g plant^-1) and 40 NH₄NO₃ g plant^-1. Effects on yields were analysed through three options: 1) Increased salinity (S); 2) Sum of added salt and nitrogen (N+S); 3) Nitrogen / Salts ratio (N/S). It was concluded the main several aspects: 1) Until a maximal accumulative NH₄NO₃ amount of 15 g plant^-1, there was always a N-S combination effects; on the other hand, beyond this value, osmotic pressure, rather than N-S combination is affecting the yield; 2) Maximal yield was obtained at 50 g of accumulative NaCl and 19 g of accumulative NO ₃NH₄ g plant^-1; beyond these values, osmotic pressure rather than N-S combination is affecting the yield, due to the lower N/S ratio.